A reed stop of the Regal class:
a particularly thin-toned Vox Humana.
Grove translates Jungfernregal as “like a girl's voice”,
and Williams attributes the name to the pitch: 4' or 2' in the pedal.
Audsley suggests that the stop originally appeared in a portative
organ, which was played by young maidens.
Wedgwood conjectures that “the instrument was used to accompany
the Angelus, a hymn to the [Blessed Virgin Mary]”.
Praetorius (by way of Adlung) describes it as follows:

Jungfrauenregal or [Jungfrauenregal]bass is at 4' pitch,
a small open Regal with a small, slight body,
about one or at the most two inches tall.
It gets its name from the fact that if it is used with other stops,
flute stops in the pedal, it sounds just like a
maiden's voice singing in the bass register.

Adlung considers Geigenregal and
Singendregal to be synonyms, and
suggests that Jungferstimme may be as well.
The spelling Junfernregal, listed by Audsley,
may be a misspelling.
The name Virginal is given only by Wedgwood.
Grove dates Jungfernregal from around 1625.